The present invention relates to a liquid crystal display apparatus or use in portable personal computers, liquid crystal televisions, personal digital assistants, cellular phones, etc. and an edge-lighting planar light source device used therein.
Recently, a color liquid crystal display apparatus has been widely used in various applications such as in portable personal computers, portable liquid crystal televisions or video built-in type liquid crystal televisions having a color liquid crystal display panel, personal digital assistants, cellular phones, etc. In addition, the amount of information to be processed increases, a demand on the liquid crystal display apparatus is diversified and it becomes necessary to cope with multimedia. Accordingly, there has been energetically promoted to provide the liquid crystal display apparatus having large-sized and super fine displaying panel.
The liquid crystal display apparatus comprises mainly a back light and a liquid crystal display device. An under-lighting system in which a light source is disposed just under the liquid crystal display device and an edge-lighting system in which a light source is disposed so as to face a side edge surface of a light guide are used as a lighting system of the back light. Recently, the edge-lighting system has been more frequently used because it is more suitable for reducing the size of the liquid crystal display apparatus. In the edge-lighting system, the light source is disposed so as to face the side edge surface of a planar light guide so that the light is emitted from the entire principal surface of the light guide to provide a planar light source device.
Such apparatuses as portable personal computers, liquid crystal televisions, etc. in which the color liquid crystal display apparatus is used as a component are driven by battery for portable use. The power supplied by the charged battery is consumed mainly by the liquid crystal display apparatus, and especially the rate of power consumption of the planar light source device used in the liquid crystal display apparatus is significant. Therefore, in order to prolong the operation time duration of the liquid crystal display apparatus by the charged battery so as to enhance utilization of the apparatus, it is important to reduce the power consumption of the planar light source device as much as possible. However, it is not preferable to lower the luminance of the planar light source device by suppressing the power consumption of the device, because it becomes hard to observe the liquid crystal display.
In JP(B)-7-27137, there is disclosed a planar light source device provided with a light guide having a light emitting face of uniformly roughened surface and a prism sheet having a plurality of elongated prisms formed on at least one surface thereof and being disposed on the light emitting face side of the light guide, so that the power consumption of the planar light source device is reduced without lowering the luminance thereof to thereby improve optical efficiency of the device. In such a planar light source device, there is a problem that distribution of the emitted light becomes extremely narrow to restrict the range of viewing angle, especially in a plane perpendicular to the linear light source, although the luminance becomes higher. In addition, there is another problem that it is difficult to apply the planar light source device to large size liquid crystal display apparatus, etc., although the device is suitable for small size liquid crystal display apparatus, etc. which does not necessitate wide range of the viewing angle.
In JP(A)-5-72532, JP(A)-7-199116, JP(A)-8-179322, etc., there is disclosed a planar light source device provided with a light guide in which a plurality of elongated prisms each having a triangular cross section are formed on at least one surface thereof so as to extend in perpendicular to the light incident face, so that substantially uniform, highly luminous light emitting face is formed to disperse the light entered in the light guide. However, in such a planar light source device, it is not possible to sufficiently narrow the distribution of the emitted light in a plane parallel to the light incident face of the light guide, and thus the planar light source device having higher luminance cannot be obtained. In addition, there is a problem in the device that the luminance within the range of the viewing angle becomes lower, because the amount of light (side lobe) emitted at an angular area of greater than about xc3x9740xc2x0 relative to the normal direction of the light emitting surface of the planar light source device, that is, at an angular area without effective range of the viewing angle is relatively large.
In JP(A)-11-224516, there is disclosed a planar light source device provided with a light guide in which an uneven surface having relatively small average slant angle is formed on one of the light emitting face and the back surface opposing thereto, and a plurality of elongated prisms each having a triangular cross section are formed on the other of the light emitting face and the back surface so as to extend in perpendicular to the light incident face. In the planar light source device described as a practical embodiment in JP(A)-11-224516, uniformity of the luminance in all over the light emitting face is superior because of the directive light emitting means constituted by the uneven surface of relatively small average slant angle. However, there is a problem that the luminance is low whereas the range of the viewing angle can be increased, because the vertical angle of the elongated prisms formed on the light guide is 90xc2x0.
In JP(A)-10-282496 and JP(A)-10-282342, planar light source device provided with a light guide in which a plurality of elongated prisms each having a triangular cross section and the vertical angle of 125 to 165xc2x0 or 60xc2x0 or more are formed on the light emitting face so as to extend in perpendicular to the light incident face in order to improve viewing angle characteristic of the planar light source device. However, in the planar light source device disclosed in JP(A)-10-282496, the elongated prisms function to concentrate the emitted light in a plane parallel to the light incident face to enhance the luminance instead of one of two prism sheets used in the conventional planar light source device. Therefore, the distribution of the emitted light cannot be made broader, although the luminance can be made greater to some extent.
Therefore, an object of the present invention is to provide an edge-lighting planar light source device which is excellent in uniformity of luminance within a light emitting face of the light guide, relatively broad in distribution of the emitted light to thereby realize a desired range of viewing angle without lowering the luminance, and high in efficiency of light utilization.
A planar light source device according to the present invention is characterized in comprising a light source; a light guide which has a light incident face on at least one side end surface thereof which confronts the light source, and a light emitting face on one of two principal surfaces thereof; and a light direction varying sheet which is disposed on the light emitting face of the light guide, wherein a back surface of the light guide opposing to the light emitting face is formed as a prism surface which comprises a plurality of elongated prisms each having substantially triangular cross section of a vertical angle within a range of 60 to 80xc2x0 or 125 to 150xc2x0 so as to extend in parallel to each other and in perpendicular to the light incident face, and a major part of the light emitting face is formed as an uneven surface having an average slant angle of 0.3 to 30xc2x0.
A liquid crystal display apparatus according to the present invention is characterized in comprising the above planar light source device and a liquid crystal display device disposed so that the planar light source device illuminates the liquid crystal display device.